Ion implantation is a standard technique for introducing conductivity-altering impurities into workpieces such as semiconductor wafers. In a conventional beamline ion implanter, an ion source generates an ion beam and extraction electrodes extract the beam from the source. An analyzer magnet receives the ion beam after extraction and filters selected ion species from the beam. The ion beam passing through the analyzer magnet then enters an electrostatic lens comprising multiple electrodes with defined apertures that allow the ion beam to pass through. By applying different combinations of voltage potentials to the multiple electrodes, the electrostatic lens can manipulate ion energies. A corrector magnet shapes the ion beam generated from the electrostatic lens into the correct form for deposition onto the workpiece. A deceleration stage comprising a deceleration lens receives the ion beam from the corrector magnet and further manipulates the energy of the ion beam before it hits the workpiece. As the beam hits the workpiece, the ions in the beam penetrate the surface of the workpiece coming to rest beneath the surface to form a region of desired conductivity.
In the operation of the conventional ion implanter, it is usually necessary to measure the cumulative ion dose implanted in the workpiece and the dose uniformity over the surface area of the wafer. A Faraday cup is one type of device used to measure the cumulative ion dose implanted in the workpiece as well as the dose uniformity over the surface area of the wafer. In operation, the Faraday cup, positioned in the vicinity of the target workpiece, receives the ion beam at selected intervals of the ion implantation. The ion beam passes into the Faraday cup and produces an electrical current which is representative of ion beam current. The Faraday cup supplies the electrical current to an electronic dose processor, which integrates the current with respect to time to determine the cumulative ion dose. The dose processor may be part of a feedback loop that can control the ion dose of the ion implanter. Faraday cups may also be used to monitor beam current at other locations along the beam line.
One type of Faraday cup used with an ion implanter has a cup body that defines a chamber which has an entrance aperture to receive the ion beam. This type of Faraday cup has a suppression electrode positioned in proximity to the entrance aperture to produce electric fields for inhibiting stray ions from entering the chamber. Also, the Faraday cup has a magnet assembly positioned to produce magnetic fields that not only inhibit the escape of electrons originating in the chamber, but also inhibit stray electrons from entering the chamber.
A problem that can rise from using such a Faraday cup with the ion implanter is that shifts in ion dose can occur when the ability of the magnet assembly to inhibit or suppress stray electrons from escaping the chamber has been compromised. A shift in ion dose can result in workpieces not having the desired conductivity and sometimes may lead to scrapping of workpieces. Errors in monitored beam current can also occur if the magnetic suppression is compromised. If the magnetic field associated with the magnet assembly in the Faraday cup were monitored, it is likely that shifts in ion dose or errors in monitored beam current could be detected earlier and corrected before serious doping problems arose that would necessitate scrapping workpieces. Because there are several magnetic fields present in a typical ion implanter, some of which are larger than the Faraday cup field and vary from recipe to recipe, it is difficult to distinguish between the magnetic field attributed to the magnet assembly in the Faraday cup and stray magnetic fields that arise from the analyzer magnet, corrector magnet and other sources in the vicinity that may produce a magnetic field. As a result, there are no approaches available that provide a reliable methodology for monitoring the magnetic field associated with the magnet assembly in the Faraday cup.